Conventionally, an adhesive resin composition (hereinafter referred to as "adhesive composition") is generally classified as either a hot-melt adhesive composition or a reactive one.
It is well known in the art that hot-melt adhesive composition has satisfactory working and instantaneous adhesive properties, because it exhibits increased instantaneous adhesive force when it is applied using a hot-melt applicator after being melted by heating and then solidified by cooling. However, its adhesive power deteriorates at an elevated temperature, particularly at a temperature of 80.degree. C. or higher.
Compositions for the hot-melt adhesive are typically disclosed in Japanese Patent Application (OPI) Nos. 25033/1974 and 90342/1976, Japanese Patent Publication No. 17514/1983, and Japanese Patent Application (OPI) Nos. 67772/1983, 147473/1983, 68385/1984, 158276/1982, and 28476/1985, and they include an ethylene-vinyl acetate resin composition; a polyolefin resin composition, such as a low density polyethylene resin composition or an atactic polypropylene one; a block polymer resin composition, such as an SIS resin composition or an SBS one, an SEBS one or the like; a butyl rubber resin composition; a polyester resin composition; a polyamide resin composition; and the like.
The hot-melt adhesive composition is widely applied to a variety of fields, including bookbinding, packaging, fabrics, furnitures, woodworking, light electrical appliances, transportation, and the like. However, it has a restriction on heat-resistant adhesion, so that it is merely recognized as a non-structural adhesive.
An adhesive composition exhibiting increased heat-resistance, wherein initial adhesion obtained at a state of hot-melt is kept effective, is desired, particularly in the field of product assembly. Unfortunately, the conventional hot-melt adhesive composition is not directed to such applications because it is highly deteriorated in adhesion at a high temperature.
The reactive adhesive composition is used as a structural adhesive, because it exhibits rigidity and adhesive strength at a high temperature. However, the reactive adhesive compositions widely known in the art, such as an epoxy adhesive composition, a urethane one, an acrylic one, or the like, are highly deteriorated in initial adhesion; therefore them is required to provide it with adhesion through a curing reaction. Unfortunately, this causes the period of time required for curing to be significantly increased.
In view of the foregoing, much effort has been directed to development of a reactive hot-melt adhesive composition which has characteristics of both the conventional hot-melt adhesion composition, such as initial adhesive properties and the like, and characteristics of the conventional reactive heat-resistant adhesive composition. For example, Japanese Patent Publication Nos. 518/1972 and 30898/1976 each disclose a reactive hot-melt adhesion composition that comprises ethylene-vinyl acetate resin, one kind of specified urethane prepolymer, and tackifier resin.
Also, Japanese Patent Application (OPI) Nos. 37936/1977 and 123436/1977, and Japanese Patent Publication No. 45954/1981, each disclose another reactive hot-melt adhesive composition, which comprises any one of polyethylene resin, polyester resin, ethylene-vinyl acetate resin, and ethylene-ethylacrylate resin, plus one kind of specified urethane prepolymer, and tackifier resin.
A further reactive hot-melt adhesive composition is disclosed in Japanese Patent Application (OPI) No. 120785/1988, which comprises a thermoplastic resin ingredient that mainly consists of a styrene block copolymer and a specified urethane prepolymer. The adhesive composition has initial adhesive properties provided by the thermoplastic resin ingredient based on the styrene block copolymer, toughness provided by the styrene block copolymer, and heat-resistant adhesive properties and elasticity provided by a cured urethane prepolymer comprising a combination of an unsaturated hydrocarbon polyol and polyoxybutylene glycol. The reactive hot-melt adhesive composition thus prepared exhibits initial adhesion through the thermoplastic resin acting as a base polymer, and it provides rigidity at an elevated temperature by urethane reaction that occurs over time. Nevertheless, the adhesive composition generally fails to exhibit satisfactory initial adhesive properties and heat-resistance.
Still another reactive hot-melt adhesive composition is disclosed in each of Japanese Patent Application (OPI) Nos. 181375/1987 and 14287/1989, which composition comprises a polyurethane prepolymer that is solid or crystalline at a normal temperature. Unfortunately, the composition is decreased in initial adhesive properties and substantially varied in melting viscosity during coating.
Thus, there has not yet been developed a reactive hot-melt adhesive composition of the moisture-curing type that is capable of satisfying demands in the market. Accordingly, it is highly desirable to provide a reactive hot-melt adhesive composition that minimizes the variation in melting viscosity prior to moisture-curing; that exhibits good hot-melt coating properties; that provides tackiness at a normal temperature; that ensures an increased pot life at a normal temperature; that provides satisfactory initial adhesive properties; and that exhibits increased heat-resistance after moisture-curing.
An acrylic pressure-sensitive adhesive is directed to various applications, such as pressure-sensitive labels, pressure-sensitive sheets, pressure-sensitive tapes, and the like, because of its improved weather-resistance, resistance to deterioration and stickiness. The acrylic pressure-sensitive adhesives that are currently widely used include solvent-type acrylic pressure-sensitive adhesives and emulsion-type ones.
For a hot-melt pressure-sensitive adhesive, a composition containing a styrene-isoprene-styrene block copolymer as a base polymer is currently used. However, the adhesive is deteriorated in weather-resistance and resistance to deterioration.
Some attempts have been made to develop a hot-melt-type acrylic pressure-sensitive adhesive as the demand for a non-solvent-type pressure-sensitive adhesive has increased. However, an acrylic hot-melt pressure-sensitive adhesive disclosed in, for example, each of Japanese Patent Application (OPI) Nos. 75975/1984, 125774/1983, 161484/1981, and 315409/1989, generally fails to exhibit satisfactory hot-melt coating properties and cohesion.
In order to eliminate the foregoing disadvantage, a hot-melt pressure-sensitive adhesive of the type that is cured by activation energy beams is proposed, as disclosed in Japanese Patent Application (OPI) Nos. 276879/1990, 119082/1991 and 220375/1991. Unfortunately, the proposed adhesive has a disadvantage that when post-crosslinking is carried out using electron beams in order to increase cohesion of the adhesive, specific equipment is needed for the irradiation of electron beams, resulting in on-site workability being deteriorated.
A moisture-curing hot-melt adhesive composition is disclosed in Japanese Patent Application (OPI) No. 259984/1991, which comprises alkyl (metha)acrylate, macromer (polymer-like monomer), (metha)acrylate having a silyl group, and an isocyanated compound. However, the adhesive composition has a drawback in hot-melt coating prior to moisture-curing. In particular, workability is substantially restricted by toxic gas produced from the isocyanate compound during melting at a high temperature.
In this specification and claims "(metha)acrylate" and "(metha)acrylic" mean acrylate or methacrylate and acrylic or methacrylic.
A moisture-curing adhesive composition is disclosed in Japanese Patent Application (OPI) No. 139584/1991, which comprises alkyl (metha)acrylate, a polymerizable polymer, and unsaturated isocyanate. The adhesive composition has a disadvantage in that its melting viscosity is substantially varied. In order to improve the disadvantage, a moisture-curing hot-melt adhesive composition which comprises alkyl (metha)acrylate, a polymerizable polymer, unsaturated isocyanate, an organic tin compound, and an organic phosphoric compound is proposed, as disclosed in Japanese Patent Application (OPI) No. 31482/1992. However, addition of an organic phosphoric compound for restricting the variation in melting viscosity causes moisture-curing of the adhesive to be delayed. Moreover, Japanese Patent Application (OPI) Nos. 139584/1991 and 31482/1992 each disclose a graft copolymer. However, sole use of the graft copolymer does not exhibit sufficient initial adhesive properties and initial cohesion prior to moisture-curing when the melting viscosity is adjusted to an appropriate level (100,000 cps or less) at a relatively low temperature (about 120.degree. C.) in order to provide the copolymer with satisfactory hot-melt coating properties.
Thus, there has not yet been provided an acrylic hot-melt pressure-sensitive composition that satisfies demands in the market. Accordingly, it is highly desirable to develop a hot-melt adhesive composition that minimizes the variation in melting viscosity prior to moisture-curing; that exhibits good hot-melt coating properties; that provides satisfactory initial cohesion; and that exhibits increased heat-resistant cohesion after moisture-curing.